1. Field of the Invention
The present invention relates to ionization of particles residing at various surfaces. In particular, the invention concerns a novel ionization method for the purposes of mass spectrometry (MS). In addition, the invention concerns ionization systems.
2. Related Art
An important recent development in mass spectrometry is the capability for direct desorption/ionization of analytes from surfaces at ambient conditions. In ambient ionization methods, ions are generated from a sample outside the MS and sampled into the MS without prior sample preparation or separation. Analysis of compounds from both artificial and native surfaces is fast. Several atmospheric pressure direct ionization methods have recently been described, namely                desorption/ionization on silicon (DIOS) [Wei et al, Nature 1999, 399, 243-246 and Laiko et al, Rapid Commun. Mass Spectrom. 2002, 16, 1737-1742],        desorption electrospray ionization (DESI) [WO 2005/094389 and Takats et al, Science 2004, 306, 471-473],        desorption atmospheric pressure chemical ionization (DAPCI) [US 2007/0187589 and Takats et al, Chem. Commun. 2005, (15), 1950-1952],        direct analysis in real time (DART) [Cody et al, Anal. Chem. 2005, 77, 2297-2302],        electrospray-assisted laser desorption/ionization (ELDI) [Shiea et al, J. Rapid Commun. Mass Spectrom. 2005, 19, 3701-3704],        atmospheric solids analysis probe (ASAP) [McEwen et al, Anal. Chem. 2005, 77, 7826-7831], and        desorption sonic spray (DeSSI) [Haddad et al, Rapid Commun. Mass Spectrom. 2006, 20, 2901-2905].        
Among the wide range of applications of these techniques are the analysis of explosives, pharmaceuticals, metabolites, proteins, polymers, and drugs of abuse from different surfaces and matrices such as glass, paper, plastics, blood, urine, tablets and ointments. Moreover, the techniques have been applied to discovery of natural products from plant material, imaging of biological tissues, and detection of analytes after separation by thin-layer chromatography (TLC) or ultra-thin layer chromatography (UTLC).
Of the ambient ionization techniques listed above, DESI and DAPCI are most relevant in this context. In DESI, an electrospray source creates charged plume of droplets that are directed at a solid sample a few millimeters to a few centimeters away. The charged droplets pick up the sample through interaction with the surface and then form highly charged ions that can be sampled into a mass spectrometer. DAPCI, on the other hand uses combination of a flow of solvent vapor and a corona discharge to ionize the sample.
The above ambient ionization techniques introduced thus far work efficiently when the analytes are polar and easily protonated or deprotonated. However, neutral and nonpolar analytes are usually ionized less efficiently or not at all.
Atmospheric pressure photoionization (APPI), which was introduced several years ago as a gas-phase ionization technique for liquid chromatography-mass spectrometry (LC-MS) [Robb et al, Anal. Chem. 2000, 72, 3653-3659 and Syage et al Am. Lab. 2000, 32, 24-29], has expanded the range of analytes that can be analyzed by LC-MS to neutral and nonpolar molecules. Ionization in APPI is initiated by photons emitted by a krypton discharge lamp that photoionizes analytes directly or indirectly through gas-phase reactions with photoionized dopant molecules. Since photoionization is dependent on the ionization energies of both the analytes and the dopant, compounds with low proton affinities that are usually not ionized in electrospray or atmospheric pressure chemical ionization (APCI) can be ionized in APPI. However, the conventional APPI method is used only for liquid-form samples, implying that the samples, if not inherently in liquid phase, must be dissolved. This requires a lot of preparation work by laboratory personnel and sets limitations for the selection of samples that can be used in connection with APPI.